1. Field of the Invention
The present invention relates to an automotive electric power steering device in which an electric motor is used to supplement steering torque generated by the driver's steering force.
2. Description of the Related Art
In the field of electric power steering devices, various approaches have been suggested to restrict the upper limit for indicator current to an auxiliary electric motor under a predetermined condition, so that the auxiliary electric motor is not overheated by overload.
FIG. 3 is a diagram showing an example of a method of controlling indicator current to a motor by a conventional electric power steering device (see for example JP-A-2002-370660). In FIG. 3, the electric power steering device includes a motor 1 that supplements the driver's steering force, a target current operation unit 2 that calculates current to be passed to the motor 1 for supplementing the driver's steering force, and a motor current control unit 3 that calculates a motor driving signal for the target current from the target current operation unit 2 and controls the motor current.
The motor current control unit 3 includes a motor current detecting circuit 4 that detects the motor current, a comparator 5 that compares a current command from the target current operation unit 2 and an output from the motor current detecting circuit 4, and motor driving signal operation means 6 for calculating a motor driving signal in response to the output received from the comparator 5 so that the current deviation is nullified and thus controlling the motor current.
The motor target current operation unit 2 includes a first target current operation unit 7 that calculates a first target current determined based on detection torque from steering torque detecting means 10 and outputs from handle angle information detecting means 11 and vehicle speed detecting means 12, motor current limiting value operation means 8 for preventing the motor and the controller from being overheated because of motor overload based on inputs from the vehicle speed detecting means 12, ambient temperature detecting means 13, and the motor current detecting means 4 in the motor current control unit 3, and the motor current limiting means 9 in the motor current control unit 3, and motor current limiting means 9 for keeping the output of the first target current operation unit 7 below the output of the motor current limiting value operation means 8.
In this example, the motor current limiting value operation means 8 uses an ambient temperature value obtained by the ambient temperature detecting means 13 as an initial value and thereafter limits the motor current based on an ambient temperature estimation value obtained by shifting the ambient temperature value to the lower temperature side as the vehicle travels for a prescribed period or more and the quantity of heat generated from the motor and the controller estimated by adding up outputs from the motor current detecting circuit 4.
As described above, in the disclosure of JP-A-2002-370660, the device includes the motor current limiting value operation means 8, and the current limit for the motor indicator current value is determined based on a heat quantity estimation value obtained by adding up motor currents from the motor current detecting means 4 and an ambient temperature obtained from the ambient temperature detecting means 13. If the ambient temperature obtained from the ambient temperature detecting means 13 is high and the motor and the controller are located in the vehicle interior, the current limiting is eased in expectation of a decrease in the vehicle interior temperature after travelling for a prescribed period or more.
However, if a vehicle does not have such means for detecting the ambient temperature, the case of the maximum temperature in use must be assumed, and the current can be limited excessively, which lowers the supplementing effect by the motor. Even with the means for detecting the ambient temperature, it would be difficult to obtain accurate ambient temperature because of the effect of heat generated from the motor after current is passed to the motor. If the motor and the controller are located outside the compartment, an air stream created by the running vehicle or heat from the engine can greatly affect the ambient temperature.